The Testing Allocator Catches Leaks
Detect memory bugs automatically.
The Testing Allocator Catches Leaks is a free Zig Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Zig Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Memory Bugs Are Sneaky
A program can pass every value check yet still leak memory by forgetting to free it. Zig gives tests a tool to catch exactly that.
Meet testing.allocator
Inside a test you get a special allocator at std.testing.allocator. Use it anywhere your code needs to allocate heap memory.
const a = std.testing.allocator;It Tracks Every Allocation
The testing.allocator remembers every block it hands out. When the test ends it checks whether each one was given back.
A Leak Fails the Test
If even one allocation is never freed, the testing.allocator reports a leak and the test fails, pointing you at the missing free.
Allocate in a Test
Ask the allocator for memory just like in real code. Here alloc reserves a buffer of ten bytes that you must release later.
const buf = try a.alloc(u8, 10);Free with defer
Pair each allocation with a defer that frees it. The cleanup is scheduled immediately, so it runs even if the test fails partway.
const buf = try a.alloc(u8, 10);
defer a.free(buf);Forget free, Catch the Bug
Drop that defer and the test fails with a leak report. This is how the allocator turns a silent bug into a loud, fixable failure.
It Also Spots Double-Frees
The same checked allocator catches freeing the same block twice or freeing memory it never gave you, which are classic crashes.
Pass It Into Your Code
Functions that allocate take an allocator parameter. In a test, hand them testing.allocator so their memory is tracked too.
const list = try buildList(std.testing.allocator);Clean Up What You Build
Whatever your function allocated, the test must release. Add a defer right after the call to free or deinit the returned data.
defer list.deinit();Free Memory Testing
Because leaks fail the build, every passing test proves its memory was balanced. That confidence is unique to testing.allocator.
Quick Check
You run a test that allocates with std.testing.allocator but never frees the buffer. What happens?
Recap
You used std.testing.allocator to catch leaks and double-frees for free. Pair each alloc with a defer free and your tests stay clean. ✅
Frequently asked questions
Is the “The Testing Allocator Catches Leaks” lesson free?
Yes — the full text of “The Testing Allocator Catches Leaks” is free to read here on the web, and the Zig Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Zig Academy course, upgrade to CoddyKit PRO.
What will I learn in “The Testing Allocator Catches Leaks”?
Detect memory bugs automatically. You practise Zig Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Zig Academy?
No prior experience is required. Zig Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “The Testing Allocator Catches Leaks” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Zig Academy lesson?
Yes. Every Zig Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Writing test Blocks
- Assertions with std.testing
- The Testing Allocator Catches Leaks
- Running and Filtering Tests